1. Field of the Invention
The present invention relates to an apparatus and method for canceling interference in a Broadband Wireless Access (BWA) system, and, in particular, to an apparatus and method for canceling interference of neighboring cells in a multi-cell based BWA system.
2. Description of the Related Art
In general, voice services have been a primary concern in the development of communication systems. In addition to voice service, provision of various multimedia services as well as data services is becoming important when developing communication systems. However, a voice-based communication system fails to satisfy user demand due to a relatively small transmission bandwidth and an expensive service fee. Moreover, the advance of communication technologies and the growth of demand for Internet services have resulted in an increased need for a communication system capable of effectively providing the Internet services. To cope with such user demand, a Broadband Wireless Access (BWA) system has been introduced for effective provision of broadband Internet services.
In addition to voice services, the BWA system supports various data services with a high or low speed as well as multimedia application services (e.g., high-quality moving pictures). The BWA system can access a Public Switched Telephone Network (PSTN), a Public Switched Data Network (PSDN), an Internet network, an IMT2000 network, and an Asynchronous Transfer Mode (ATM) network in a fixed or mobile environment based on a wireless medium using a broadband spectrum (e.g., 2 GHz, 5 GHz, 26 GHz, 60 GHz, etc.). Furthermore, the BWA system can support a channel transfer rate of 2 Mbps or more. According to mobility of a Mobile Station (MS) (i.e., whether it is moving or fixed), communication environment (i.e., indoor or outdoor), and a channel transfer rate, the BWA may be classified into a broadband wireless subscriber network, a broadband mobile access network, and a high speed wireless Local Area Network (LAN).
A wireless access method of the BWA system is standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.16 group.
According to the IEEE 802.16 standard, due to a wide bandwidth, larger sized data can be transmitted at a short period of time compared with the conventional wireless technique for a voice service. In addition, a channel (or resource) can be shared by all MSs, resulting in effective channel use. Moreover, since a Quality of Service (QoS) is ensured, the MSs can receive different QoSs on the basis of service features.
In the IEEE 802.16 communication system, an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme is used for a physical channel. That is, the BWA system employing the OFDM/OFDMA method transmits a physical channel signal by using a plurality of sub-carriers, and thus high-speed data transmission can be achieved.
With a multi-cell structure, the BWA system can support mobility of an MS, and all cells can use the same frequency for the effective use of frequency resources. However, performance of such a multi-cell based system is significantly affected by interference of neighboring cells.
FIG. 1 illustrates a multi-cell based BWA system.
Referring to FIG. 1, MS 110 is located in a cell overlap region in a multi-cell environment having a frequency reuse rate of 1. MS 110 transmits an interference signal to a Base Station (BS) 1 located in a neighboring cell. The interference signal affects a signal of an MS 112 located within the cell, resulting in deterioration of demodulation performance. Therefore, a method of canceling neighboring cell interference is necessary in such a multi-cell based system.
FIG. 2 illustrates a multi-cell Up-Link (UL) environment when neighboring cell interference occurs.
Referring to FIG. 2, in practice, neighboring cell interference is caused by a plurality of cells (or MSs). A conventional method of canceling multi-cell interference in such an environment will now be described.
One example of the conventional method of canceling interference is based on a Minimum Mean Square Error (MMSE) scheme.
The MMSE based method can be expressed as Equation (1) below.{circumflex over (x)}=WYW=(HD)HR−1  (1)                Herein, Y denotes an input signal, and is an NR×1 column vector when the number of receiving antennas is NR. W denotes an MMSE filter coefficient, and is a 1×NR row vector when the number of signals to be demodulated is 1. {circumflex over (x)} denotes a signal estimated using an MMSE filter. HD denotes a wireless channel feature of a signal to be demodulated, and is an NR×1 column vector. R denotes a correlation of an input signal, and is an NR×NR matrix. Exponential −1 denotes an inverse matrix. Exponential H denotes a Hermitian Transpose.        
In Equation (1), the correlation matrix R is expressed as Equation (2) below.
                                                        R              =                              E                ⁡                                  [                                      YY                    H                                    ]                                                                                                        =                              E                ⁡                                  [                                                            (                                              HX                        +                        N                                            )                                        ⁢                                                                  (                                                  HX                          +                          N                                                )                                            H                                                        ]                                                                                                        =                                                HH                  H                                +                                                      σ                    2                                    ⁢                  I                                                                                        (        2        )            
Herein, X denotes a vector of signals transmitted from all MSs, and is an NU×1 column vector when the number of MSs is NU. H denotes a wireless channel matrix between MSs and receiving antennas, and is an NR×NU matrix. N denotes noise of a receiving antenna, and is an NR×1 column vector. σ2 denotes noise power. I denotes an NR×NR identity matrix.
As described above, the conventional method of canceling interference using the MMSE scheme has a problem in that channel estimation has to be carried out not only for a channel of a desired MS but also for a channel of a neighboring cell producing an interference signal. This case makes it difficult to determine the number of valid interference signals. Furthermore, channel estimation for an interference signal is obtained through a complex computation process. In addition, since the interference signal has a smaller magnitude than a desired signal to be demodulated, accuracy of channel estimation is not guaranteed, thereby deteriorating demodulation performance.
Accordingly, due to complexity and inaccuracy of computation, there has been difficulty in the use of the conventional method of canceling interference using the MMSE scheme.